The present invention relates to improved release agents used in processes for preparing molded articles. More particularly, the invention relates to externally-applied wax-based release agents which are used in processes for preparing molded articles such as oriented strand board, waferboard or plywood.
An advantage of the release agents of the present invention is that they possess a unique combination of viscosity, crystallinity and hardness properties which heretofore were not believed to be attainable in single component wax-based released agents.
It is known in the art that molded lignocellulosic or wood-based products can be prepared using standard batch procedures or, more recently, using continuous techniques. The preparation of wood composites typically includes introducing an aqueous composite mixture of wood chips or the like between two heated metal plates which press and shape the composite mixture under pressure. Furthermore, the use of wax release agents in processes for the binding of lignocellulosic materials with polyisocyanates has also been previously described. See, for example, WO 98/00464, the contents of which are incorporated herein by reference.
In a typical process, an organic isocyanate binder such as a polyisocyanate, which is usually in the form of a solution or an aqueous emulsion, is applied to a lignocellulosic material which is then subjected to heat and pressure. The quality of the resulting articles, e.g. sheets or molded bodies such as chipboard or plywood, is largely due the adhesive properties of the organic polyisocyanates. Unfortunately, these adhesive polyisocyanates are the cause of significant problems during the molding process. For example, isocyanates often cause severe sticking of the lignocellulosic material to the hot metal press plate surfaces it is in contact with during the hot-pressing operation. Consequently, the molded product is often harmed as a result of trying to remove it from the press plates. Another problem associated with polyisocyanates is that the time required for cleaning the contaminated hot metal surfaces can be substantial. Thus, in spite of the usefulness of polyisocyanates as adhesive binders, they are associated with drawbacks which can be costly.
Alternative adhesive or binder products such as urea formaldehydes, phenol formaldehydes and phenolic impregnated papers have been used as replacements for polyisocyanates. Such alternative binders, however, have not met with success for various reasons. For example, the moisture resistance associated with the formaldehydes is less than that which is desirable. Furthermore, these products are associated with certain health risks. Interest has therefore been focused on developing "release agents", that is, additives specifically intended to favor the release of the manufactured articles from the metal plates of the press and overcome other processing difficulties associated with polyisocyanates.
Often release agents are combined with the polyisocyanates as part of the binding emulsion. Such agents are typically referred to as internal release agents. Alternatively, external release agents which are usually applied to the press surfaces which come into contact with the lignocellulosic composite mixture have been suggested. For example, conventional release agents such as oils, wax polishes, metallic soaps, silicones and polytetrafluoroethylene have been applied externally on the metal surfaces but have proved unsatisfactory.
The shortcomings associated with currently available release agents which can be used with isocyanates are particularly evident in the context of commercial scale, continuous or semi-continuous molding operations. The press equipment is used repeatedly in short cycle times. If one wishes to rapidly produce articles having uniform quality, the problems associated with adhesion to the press parts must be essentially eliminated. Furthermore, the time required for cleaning the press parts surfaces to remove resinuous material formed on the hot surfaces of the press parts due to thermal decomposition must be reduced to a minimum.
It has thus been determined that there is a continued need for improvements in release agents. Specifically, there is a need to provide release agents which have a desirable combination of several physical attributes, such as viscosity and hardness, to overcome the shortcomings associated with isocyanate-based binding systems. For example, some external release agents such as emulsions of A-C.RTM. 6702 wax, a product of AlliedSignal, Morristown, N.J., have good release properties and do not cause a build up on the forming belts. These emulsions, however, may leave a sticky residue on many of the surfaces surrounding the spinning disk equipment which applies the emulsion to the platen and cause clogging. Emulsions of A-C.RTM. 659 wax, another AlliedSignal product, on the other hand do not leave a residue near the spinning disk equipment. The dried emulsion is deposited as small, white particles that are easily removed with compressed air. Continuous use of these release agent emulsions, however, leads to a considerable build up of wax and rosin on the forming belts, a problem often referred to as a plate-out. In sum, currently available release agents have some desirable properties and some undesirable properties. Some have sufficient hardness to form depositions around the spinning disk equipment which are easily removable, but the viscosity of these agents is too high to be absorbed by the wood. Conversely, if the release agent has a viscosity which is low enough to be absorbed by the wood, and thereby solve the problem of plating-out, it is so soft that it leaves sticky deposits in the areas surrounding the spinning disks.
It has now been found that by co-oxidizing blends of certain types of wax and wax-like release agents, the resultant products have a synergistic combination of physical attributes which are especially well-suited for use in externally-applied release agent emulsions.